The invention relates to an electric heating module for heating an air flow. This heating module is particularly provided for heating air flow in vehicles, such as motor vehicles or air planes. It essentially comprises at least one PTC—heating element and at least one heat dissipating area with heat conduction ribs, adjacent thereto and allowing air to flow through it, said ribs being effectively connected to the PTC heating element. The PTC heating element is combined with the heat conducting ribs using a frame to form a module.
PTC elements are semi-conductor resistors made from ceramics, with their active resistance depending on the temperature. The resistor—temperature characteristic does not progress linearly: the resistance of a PTC heating element first reduces only slightly with the temperature of the component rising, in order to subsequently rise rapidly at a characteristic temperature (reference temperature). This overall positive progression of the resistance—temperature characteristic (PTC=positive temperature coefficient) leads to the PTC heating element having self-controlling features. When the temperature of the component is considerably lower than the reference temperature the PTC heating element has a lower resistance so that correspondingly high current can be conducted. When a good heat dissipation from the surface of the PTC heating element is ensured, appropriately high electric power is drawn and dissipated in the form of heat. However, when the temperature of the PTC heating element exceeds the reference temperature, the PTC resistance increases rapidly so that the electric power draw is limited to a very low value. The temperature of the component then approaches an upper limit, which depends on the heat absorption of the environment of the PTC heating element. Under normal conditions, the temperature of the component of the PTC heating element can therefore not exceed beyond a characteristic maximum temperature, even when the intended heat dissipation is entirely interrupted in the case of a malfunction. This and the self-controlling features of a PTC heating element, based on which the drawn electric power is precisely equivalent to the dissipated thermal power, predestines the PTC heating element for use in heating and/or air conditioning systems of vehicles or other applications of heated air flow in vehicles. For reasons of safety, in this application field no flammable temperature may develop in the heating element, even in the case of a malfunction, although high heating output is required in normal operation.
For heating the interior compartment of vehicles it has been known to use electric heating modules having a frame, which forms a module by combining a multitude of PTC heating elements and heat dissipation areas with heat-conducting ribs adjacent thereto, allowing air to flow through. An example for such a known electric heating module is found in EP 0 350 528 A1.
In EP 1 479 918 A1 a complete ventilation module, comprising a radial fan integrated in a housing and a heating module of the type mentioned at the outset is disclosed, which is to serve as a heater for a seat in a ventilated vehicle seat. Due to the fact that a vehicle seat, for safety reasons, even in the event the ventilator malfunctions, may not exceed a maximum temperature at its surface which is tolerated by human beings, heating modules with PTC heating elements are excellently suitable, in particular because, while providing equivalent safety, they can dissipate a considerably higher heating output than conventional mats, used as heaters for seats, having electric resistance wires, with their power draw having to be very limited for safety reasons.
The previously known electric heating modules of the type mentioned at the outset generally comprise several layers of planar, side-by-side arranged PTC heating elements with their narrow sides in line with the air flow, which at their flat surfaces and their bottom sides each electrically contacting metal sheets. The adjacent heat dissipating areas are provided with meandering metal ribs, which also are positioned with their narrow sides in the air flow and at their wide side thermally contacting metal sheets of the PTC heating elements in regular intervals for heat dissipation. In order to achieve good heat transfer from the PTC heating elements to the heat conducting ribs heat conducting glue or other connection techniques can be used; however, it has been proven the most efficient solution to place the PTC heating elements and the heat conducting ribs into a frame combining them to a module and to provide inside said frame at least one spring element, which presses the alternating arranged heat dissipating areas with the heat conducting ribs against the bars with the PTC heating elements.
However, this requires a rectangular shape of the electric heating module with a cellular structure of the components, which is aerodynamically not optimal for heating air flow, in particular when the space for the respective air-flow guiding channels is very limited, like in a motor vehicle. Thus it is logical for the ventilation module for vehicle seats according to EP 1 479 918 A1 to use a radial fan. However, radial fans are known to be less suitable for this purpose because they create a high pressure with correspondingly high flow speeds.
Furthermore, the production of the known electric heating modules is hardly possible in an automated fashion due to their multi-layered, spring-loaded design within a frame. Rather, a large amount of manual labor is necessary.